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Carbohydrates
What are they?
Formula = (CH2O)n where n > 3Also called sugarMajor biomolecule in body
What do cells do with carbs?
Oxidize them for energyStore them to oxidize later for energyUse them as components of larger moleculesSugars attached to proteins and lipids
Where do carbs come from?
1. Plants “fix” CO2 from air to synthesize sugars
6CO2 + 6H2O ---> C6H12O6 + 6O2
2. Other organisms eat sugar or make sugar from smallerorganic compounds
Carbohydrates
What carbs are found in cells?
1. Monosaccharidessingle sugar
2. Oligosaccharides (disaccharides most abundant)glycosidic bondsoften linked to proteins - glycoproteinsor to lipids - glycolipids
3. Polysaccharideslarge polymersstructural - cellulose (linear)storage - starch (branched)
O
C
H
2
O
H
H
H
O
H
H
O
H
O
H
H
H
O
H
O
C
H
2
O
H
H
H
O
H
H
O
H
O
H
H
H
O
O
C
H
2
O
H
H
H
H
O
H
O
H
H
H
O
H
Maltose, a disaccharide
Glc
Glc Glc
O
C
H
2
O
H
H
H
H
O
H
O
H
H
H
O
O
C
H
2
O
H
H
H
H
O
H
O
H
H
H
O
Glc Glc nα-Amylose, component of starch
Monosaccharides
An aldehyde or ketone with > 1 hydroxyl groupColorless, crystalline, usually sweetOne _______________ group
aldose ketose
MonosaccharidesAldoses
Most common in nature - hexoses (6-carbon sugars)Components of nucleotides and nucleic acids
Six carbonsD isomers
Five carbonsD isomers
MonosaccharidesOther Aldoses
MonosaccharidesKetoses
Some exceptions to rule on D isomers onlyL-Arabinose and sugars in glycoconjugates
Monosaccharides
Monosaccharides
Epimers - Two sugars that ______________________________
MonosaccharidesWhy are these particular sugars utilized?EvolutionGlucose forms especially stable structuresMonosaccharides can spontaneously cyclize
C1 not chiral in open chain form, but it is in pyranoseStereoisomers differing only at C1 are called α and β anomersC1 is __________ carbon - carbon that used to be the carbonyl C
MonosaccharidesMonosaccharides
MonosaccharidesSugars can be reducing agents
Measure blood glc bymeasuring amount of H2O2
Fehling’s reactionCuprous ion (Cu+) forms red precipitateMeasure ppt. - estimate [glc]
reduction
oxidization
Oligosaccharides & PolysaccharidesGlycosidic bondsHydroxyl on anomeric carbon is reactive and can condense withan alcohol to form a ____________ bond
Reducing end - still has free anomeric carbonNonreducing end - no anomeric carbon
Disaccharides
_________-ases hydrolyze glycosidic bonds, specific forparticular linkages
Lactose intolerance: lack ________, the enzyme that can breakdown lactose
Occurs naturally onlyin milk
Table sugarFormed by plants, not usNo anomeric carbon
PolysaccharidesHomopolysaccharides - starch, glycogen (storage); cellulose,chitin (structure)Heteropolysaccharides - bacterial cell membrane, extracellularspace in animal cells
Polysaccharides - Storage2 types of glc polymersAmylose - linear, α1→4
Amylopectin - highly branched, α1→4 glycosidic, α1→6 branch points
Polysaccharides - Storage
Starch - α1→4 glycosidic, α1→6 branch points; 1 branch/24-30residues
Glycogen - α1→4 glycosidic, α1→6 branch points, more extensivelybranched: 1 branch/8-12 residuesnonreducing ends - glc can be mobilized quicklyLots in liver, some in muscleLiver stores glycogen equivalent to glc concentration of 400 mM(blood glc ~5 mM)
Polysaccharides - - StructureCellulosefibrous, tough, water insoluble, plant cell wall, humans can’t digestlinear, unbranched, lots of glc linked by (β1→4) glycosidic linkages
Chitintough, water insoluble, component of hard exoskeleton (insect, lobster, crab),indigestible by humanslinear, uses modified sugars (N-acetyl-D-glucosamine) with (β1→4) linkages
Polysaccharides- structureStarch - floppy left-handed helix Cellulose - rigid chain
PolysaccharidesGlycogen, starch ingested in diet hydrolyzed by α-amylases (in salivaand intestinal secretions) that break α1→4 linkages
Most animals cannot use cellulose as fuel because no enzyme tohydrolyze β1→4 linkages
Termites readily digest cellulose (wood) because their intestinal tractharbors microorganism that secretes cellulase (breaks β1→4 linkages)
Only vertebrates able to use cellulose as food are cattle and otherruminants (sheep, goats, camel, giraffes) that have extra stomachcompartment (rumen) that has bacteria/protists that secrete cellulase
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